Natural gas, found in deposits in the earth, is an abundant energy resource. For example, natural gas commonly serves as a fuel for heating, cooking, and power generation, among other things. The process of obtaining natural gas from an earth formation typically includes drilling a well into the formation. Wells that provide natural gas are often remote from locations with a demand for the consumption of the natural gas.
Thus, natural gas is conventionally transported large distances from the wellhead to commercial destinations in pipelines. This transportation presents technological challenges due in part to the large volume occupied by a gas. Because the volume of a gas is so much greater than the volume of a liquid containing the same number of gas molecules, the process of transporting natural gas typically includes chilling and/or pressurizing the natural gas in order to liquefy it, which contributes to the final cost of the natural gas.
Further, naturally occurring sources of crude oil used for liquid fuels such as gasoline and middle distillates have been decreasing and supplies are not expected to meet demand in the coming years. Middle distillates typically include heating oil, jet fuel, diesel fuel, and kerosene. Fuels that are liquid under standard atmospheric conditions have the advantage that in addition to their value, they can be transported more easily in a pipeline than natural gas, since they do not require energy, equipment, and the expense required for liquefaction.
Thus, for all of the above-described reasons, there has been interest in developing technologies for converting natural gas to more readily transportable liquid fuels, i.e. to fuels that are liquid at standard temperatures and pressures. One method for converting natural gas to liquid fuels involves two sequential chemical transformations. In the first transformation, natural gas or methane, the major chemical component of natural gas, is reacted with oxygen to form syngas, which is a combination of carbon monoxide gas and hydrogen gas. In the second transformation, known as the Fischer-Tropsch process, carbon monoxide is converted into organic molecules containing carbon and hydrogen. Those organic molecules containing only carbon and hydrogen are known as hydrocarbons. In addition, other organic molecules containing oxygen in addition to carbon and hydrogen, known as oxygenates, may be formed during the Fischer-Tropsch process. Hydrocarbons having carbons linked in a straight chain are known as aliphatic hydrocarbons that may include paraffins and/or olefins. Paraffins are particularly desirable as the basis of synthetic diesel fuel.
Typically the Fischer-Tropsch product stream contains hydrocarbons having a range of numbers of carbon atoms and thus having a range of molecular weights. Therefore, the Fischer-Tropsch products produced by conversion of natural gas commonly contain a range of hydrocarbons including gases, liquids and waxes. Depending on the product molecular weight distribution, different Fischer-Tropsch product mixtures are ideally suited to different uses. For example, Fischer-Tropsch product mixtures containing liquids may be processed to yield gasoline, as well as middle distillates. Hydrocarbon waxes may be subjected to an additional processing step for conversion to liquid and/or gaseous hydrocarbons. Consequently, in the production of a Fischer-Tropsch product stream for processing to a fuel it is desirable to maximize the production of high value liquid hydrocarbons, such as hydrocarbons with at least 5 carbon atoms per hydrocarbon molecule (C5+ hydrocarbons).
The Fischer-Tropsch process is commonly facilitated by a catalyst. Catalysts desirably have the function of increasing the rate of a reaction without being consumed by the reaction. The composition of a catalyst influences the relative amounts of hydrocarbons obtained from a Fischer-Tropsch catalytic process. Common catalysts for use in the Fischer-Tropsch process contain at least one metal from Groups 8, 9, or 10 of the Periodic Table (in the new IUPAC notation, which is used throughout the present specification). Cobalt metal is particularly desirable in catalysts used in converting natural gas to heavy hydrocarbons suitable for the production of diesel fuel. Alternatively, iron, nickel, and ruthenium have been used in Fischer-Tropsch catalysts. Nickel catalysts favor termination and are useful for aiding the selective production of methane from syngas. Iron has the advantage of being readily available and relatively inexpensive but the disadvantage of a water-gas shift activity. Ruthenium has the advantage of high activity but is quite expensive.
In addition to the catalytic metal, a Fischer-Tropsch catalyst often includes a support material. In a common method of depositing catalyst metal on a support, the support is impregnated with a solution containing a dissolved metal precursor compound and is thereafter dried. Catalyst supports for catalysts used in Fischer-Tropsch synthesis of hydrocarbons have typically been refractory oxides (e.g., silica, alumina, titania, thoria, zirconia or mixtures thereof). It has been claimed that the Fischer-Tropsch synthesis reaction is only weakly dependent on the chemical identity of the metal oxide support (see E. Iglesia et al. 1993, In: “Computer-Aided Design of Catalysts,” ed. E. R. Becker et al., p. 215, New York, Marcel Dekker, Inc.).
Dispersion of the catalytic metal on a support tends to influence the performance of a supported catalyst. Lower dispersion does not fully utilize metal sites and is not an efficient use of the available metal. In contrast, high dispersion of catalytic metal more fully utilizes metal sites and results in a higher initial catalyst activity, such as carbon monoxide conversion in the Fischer-Tropsch reaction. However, highly dispersed metal tends to be more difficult to reduce and tends to deactivate more rapidly.
Thus, it is desirable to control the dispersion of metal for supported metal catalysts. Methods for controlling dispersion typically involve a modification of the method of depositing metal on the support. For example, methods of affecting the dispersion of a catalytic metal include controlling the evaporation rate of a solvent (e.g., water) used to impregnate a support with a metal precursor (e.g., nitrates or acetates of Fe, Co, Ru, and the like), for example by varying the temperature or pressure. Other techniques for adjusting dispersion include using solvents with higher or lower boiling points than water. Further, chemical vapor deposition is a well-known technique for controlling the final dispersion of metal.
Consequently, there remains a need for supported Fischer-Tropsch catalysts having improved dispersion of catalytic metal and methods of making such catalysts.